In deep sea areas with a depth greater than 50 meters, developable wind resources are more and have higher quality, and the market prospect is broader. To develop the deep sea wind farm at these sea areas, the method of fixing various through piles at the sea bed, which is generally adopted in the offshore wind farms, has no advantage, the reason is that, as the water depth increases, the cost of a stationary platform rises perpendicularly, and the costs of construction and maintenance of the stationary offshore wind turbine are higher than those of the floating offshore wind turbine. Hence, in order to facilitate constructing the offshore wind farm in the deep sea areas, an economical and practical floating wind turbine is required to be developed. A key subject in developing a floating wind turbine is to develop a floating wind turbine foundation having a good movement performance, a compact structure and economic practicality.
The load applied on the floating wind turbine foundation used in the offshore wind power field is different from that the load applied on a mobile platform in the conventional offshore oil engineering. In addition to the combined effect from wind, waves and currents, the floating offshore wind turbine foundation also is subjected to a gyroscopic effect caused by the high-rise structure of the wind turbine, overturning moments Mx, My and a torque Mz around the vertical axis; the whole wind turbine will generate a violent movement in six degrees of freedom including axial movements along X, Y and Z axes and swings around the X, Y and Z axes, which causes a great challenge for the pitch control and yaw control system of the wind turbine, and may affect the normal operation of the wind turbine, affect the energy output, and even endanger the safety of the whole system structure.
Presently, according to the need for developing the deep sea wind farms, the deep sea floating oil platform commonly used in the sea oil industry has been employed in the wind power field, and various forms of floating wind turbines are developed, including a single pole platform (SPAR) form, a semi-submersible platform form, a tension leg platform (TLP) form and other complex forms. So far, about three floating wind turbine projects have been constructed worldwide, which are Hywind, Blue H and Windfloat. Hywind project is successfully developed by the Statoil Hydro, the Technip and the Siemens jointly and is constructed at the North Sea close to Norway in 2009. The basic concept in Hywind project is SPAR, the floating structure is an elongated steel pipe with a length of about 117 meters, and the steel pipe has one end connected to a foundation and another end connected to a wind turbine flange. Steel pipes filled by a ballast tank are transmitted to an installation site and stand on the sea surface, and the whole floating body is connected to the sea bed via three mooring anchor cables. In 2007, the Blue H Company of Holland installs a double-blade wind turbine prototype at the costal area of Italy, which uses the tension leg platform design; and the buoyancy of the platform is arranged to be greater than its gravity, the tension legs are always in a tensioned state to maintain the stability of the platform. At the end of 2011, the Principle Power, the Vestas and the EDP cooperate to complete the Windfloat project at the west coast of Portugal, the floating foundation is semi-submersible and the main body of the floating foundation is consisted by three floating cylinders, and the wind turbine stands on one of the floating cylinders; the dynamic ballast water can offset the wind heeling moment automatically, and the bottom of the wind turbine is fixed to the sea bed, with a depth greater than 50 meters, via four catenary cables.
However, the floating wind turbine projects described above generally have the following disadvantages, the costs of manufacturing, transporting and installing are high, the movement performance is not good, the structure is complicated and the service life is not long.